The Pythagoreans of ancient Greece were fascinated by whole numbers. One particular interest involved what we now call amicable numbers.
Amicable numbers come in pairs in which each number is the sum of the proper divisors of the other. The smallest such pair is 220 and 284. The number 220 is evenly divisible by 1, 2, 4, 5, 10, 11, 20, 22, 44, 55, and 110, which add up to 284; and 284 is evenly divisible by 1, 2, 4, 71, and 142, which add up to 220. The Pythagorean brotherhood regarded 220 and 284 as numerical symbols of friendship.